Smartphone-assisted colorimetric sensing platform based on manganese phosphonate MOF nanozyme for total antioxidant...

Accurate evaluation of total antioxidant capacity (TAC) is crucial for assessing the functional benefits of antioxidant-rich foods and cosmetic products. Although colorimetric sensing offers a simple and low-cost strategy for TAC determination, its practical performance depends strongly on the stability and catalytic efficiency of the chromogenic system. Herein, we report a manganese phosphonate metal-organic framework (MnP MOF), constructed from Mn²⁺ nodes and a rigid biphenyl-4,4'-diphosphonic acid ligand, as an oxidase-mimetic nanozyme for H₂O₂-free TAC determination. In contrast to the more commonly reported carboxylate- or imidazole-based MOF nanozymes, this phosphonate-based Mn MOF features strong Mn-O-P coordination and a rigid aromatic framework, which are associated with improved structural robustness and accessible catalytic sites. The MnP MOF exhibits pronounced oxidase-like activity toward 3,3',5,5'-tetramethylbenzidine (TMB) without requiring H₂O₂, thereby reducing peroxide-related instability and potential matrix interference. The material showed a low apparent K_m value of 0.32 mM and enabled sensitive detection of ascorbic acid (AA), L-cysteine (L-Cys), and glutathione (GSH), with limits of detection of 0.37, 1.02, and 1.78 μM, respectively. Combined with smartphone-based readout, the resulting platform allows rapid and portable TAC determination in fruit and cosmetic samples, demonstrating the promise of phosphonate-based MOFs for practical antioxidant sensing.